The use of natural rubber in the medical and health care industries can expose atopic individuals to latex allergens. It is believed that as much as 6% of the general population, and as high as 12% of those working in the medical profession, are latex sensitive and display allergic reactions when exposed to proteins found in natural rubber (see M. McNulty, Rubber & Plastics News Jun. 25, 2001, page. 5). The symptoms of latex allergy range from mild contact dermatitis to life-threatening anaphylaxis, which includes a rapid drop in blood pressure and difficulty breathing. Even though a heightened awareness of latex allergy now exists, the number of people becoming sensitized to natural rubber is increasing as more professions require the use of latex gloves to avoid exposure to infectious agents. It is therefore not surprising that with over 40,000 consumer products containing natural rubber (see Information from Allergy Advisor-Zing Solutions, http://allergyadvisor.com) an alternative protein free material is desired in many applications.
Although techniques exist for enzymatic deproteinization of natural rubber (see S. Kawahara, T. Kakubo, N. Nishiyama, Y. Tanaka, Y. Isono, J. T. Sakdapipanich, J. Appl. Polym. Sci. 78, 1510 (2000) and A. H. Eng, S. Kawahara, Y. J. Tanaka, Nat. Rubb. Res. 8, 109 (1993)), as well as manufacturing practices that lower the total allergens present in latex goods, currently, the most effective way to provide protein free products is to use petrochemical derived synthetic rubbers. In fact, a report from the Johns Hopkins University School of Medicine recommended that the Food and Drug Administration mandate a switch from using stoppers made with natural rubber to using all synthetic rubber medicine stoppers (again see M. McNulty, Rubber & Plastics News Jun. 25, 2001). The synthetic rubber most closely related to natural rubber (NR), is high cis-polyisoprene. Typically, this material is prepared through the use of either stereospecific titanium catalysts or with alkyl-lithium initiators. Both of these systems are effective at providing protein free synthetic polyisoprene, however, the two polymers differ greatly with respect to their micro- and macrostructure. Polymerization of isoprene with titanium tetrachloride activated with a trialkylaluminum co-catalyst results in a material with upwards of 98% 1,4-cis content (see W. Cooper, in W. M. Saltman, ed., The Stereo Rubbers, John Wiley & Sons, New York, 1977, page 48).
Polyisoprene produced commercially with alkyl lithium initiators generally does not have a cis content higher than 92%. Differences in microstructure, as well as macrostructure, allows the titanium polyisoprene (Ti-PI), but not the lithium polyisoprene (Li-PI), to display the unique advantage of strain-induced crystallization. It is the property of rapid crystallization that allows NR and Ti-PI to have high tensile strength and modulus even without the use of reinforcing fillers, a condition often found in gum stocks commonly used in the production of medical goods (see A. R. Bean, Jr., et. al., in H. F. Mark, N. G. Gaylord, N. M. Bikales, ed., Encyclopedia of Polymer Science and Technology, John Wiley & Sons, New York, Vol. 7, 1967, page. 823). Although Li-PI lacks the microstructural regularity of Ti-PI, it does have the advantage of being gel free with a narrow molecular weight distribution and linear macrostructure. These attributes allow Li-PI to display lower hysteretic properties at a lower cross-link density than NR or Ti-PI.
The use of Ti-PI and Li-PI in the medical and health care industries has gained acceptance, yet there is still a need for a number of improvements. For example, the consistency of Ti-PI is very dependent on the aluminum to titanium ratio that is used during catalyst preparation (see W. Cooper, in W. M. Saltman, ed., The Stereo Rubbers, John Wiley & Sons, New York, 1977, page 48). If the ratio drops below unity the titanium is not sufficiently reduced, causing the formation of gel in the polymer. On the other hand, if the catalyst is over-reduced, oligomers will be produced giving the material a strong odor. Both of these unwanted side reactions must be carefully controlled. Current titanium systems also suffer from inferior activity and high levels of titanium must be used resulting in elevated levels of catalyst residues and terminating agents in the finished polymer. An overall reduction in foreign substances remaining in synthetic polyisoprene is of paramount importance when the production of a clean high performance material is desired.
Li-PI is considered a clean polymer due to the use of low levels of initiator during production and lack of extractables. However, the dependence of cis content on lithium concentration leads to a polymer with very high molecular weight (see H. L. Hsieh, R. P. Quirk, Anionic Polymerization Principles and Practical Applications, Marcel Dekker, Inc., New York, 1996, p. 201). The high molecular weight, coupled with a narrow molecular weight distribution, makes processing this material difficult. Commonly a low molecular weight fraction is added to commercial material to act as a processing aid.
The use of neodymium catalyzed polyisoprene (Nd-PI), as a source of protein free synthetic natural rubber offers the combined advantages of both Ti-PI and Li-PI without their respective disadvantages. Nd-PI with a cis 1,4-content as high as 98%, gel and oligomer free, linear with a moderate molecular weight distribution, easy to process, and low in volatile and extractable residues is ideally suited for many clean high performance applications (see U.S. Pat. No. 6,871,751).
The technique of combining a neodymium salt, an aluminum alkyl, a halide source, and a diene to attain an improved result is the subject of U.S. Pat. No. 6,780,948. As the prior art describes, most any conjugated diene monomer can be used in the preforming step and each diene can be treated in the same way. For example, prior teachings imply that the contact time between the conjugated diene and the neodymium/aluminum co-catalyst treatment step is not crucial and that aging should occur after the halide source has been added. However, U.S. Pat. No. 6,780,948 explains that a minimum contact time does indeed exist for different conjugated dienes when the preparation of a completely soluble catalyst is desired. It is also crucial that this contact time occurs prior to the introduction of a halide source in order to ensure completely soluble catalyst solutions. For example, formation of a homogeneous catalyst solution is achieved when isoprene is used in the preform only if the isoprene/neodymium/aluminum alkyl solution is allowed to age for an extended amount of time prior to aluminum-chloride addition. If the first step is not allowed to proceed long enough, a precipitate is formed upon addition of aluminum-chloride. When butadiene is used in the preforming reaction this first aging period is still crucial, yet, significantly less time is needed to ensure a homogenous catalyst.
The technological advantage of a completely soluble preformed catalyst has previously been appreciated. As U.S. Pat. No. 4,461,883 teaches, a heterogeneous system is a disadvantage in an industrial setting. Likewise, U.S. Pat. No. 6,136,931 states that the use of heterogeneous catalyst systems containing suspended particles usually produces gel. This patent also states that in a heterogeneous system, compared to a homogenous one, it is more difficult to control the exact amount of catalyst added during the polymerization. Similarly, U.S. Pat. No. 6,780,948 indicates that catalyst prepared without the first aging period results in a catalyst suspension of a fine precipitate. This suspension settles upon standing into two phases. If the resulting supernate, or top layer, is used to polymerize a conjugated diene, extremely inefficient catalyst activity results. Catalyst activity can be restored in these systems only after agitation of the by-phasic mixtures. This allows for the introduction of a heterogeneous catalyst suspension to the monomer to be polymerized. However, it is now possible to ensure consistent and highly active soluble preformed catalyst formation by utilizing the appropriate two step aging technique. This is of obvious technological advantage, since there would be no need to use a stirred tank catalyst storage tank or other engineering constraints to ensure consistent catalyst suspensions.
The neodymium catalyst system prepared by the technique described in U.S. Pat. No. 6,780,948 can be used in the polymerization of isoprene monomer into polyisoprene rubber that is clear (transparent) and of high purity. U.S. Pat. No. 6,780,948 more specifically discloses a process for the synthesis of polyisoprene rubber which comprises polymerizing isoprene monomer in the presence of a neodymium catalyst system, wherein the neodymium catalyst system is prepared by (1) reacting a neodymium carboxylate with an organoaluminum compound in the presence of isoprene for a period of about 10 minutes to about 30 minutes to produce neodymium-aluminum catalyst component, and (2) subsequently reacting the neodymium-aluminum catalyst component with a dialkyl aluminum chloride for a period of at least 30 minutes to produce the neodymium catalyst system.
It is, of course, important for high purity polyisoprene rubbers to be cured using a technique that will not compromise the purity or clarity of the rubber. By the same token, it is also important for cured rubber articles made with such polyisoprene rubber to exhibit good mechanical properties. For instance, in many applications it is important for the cured polyisoprene rubber to have the highest possible elongation at break.